1. Field of the Invention
The present invention is in the field of curable powder coating resin compositions comprising a blend of aminoplast crosslinking agents and polymeric materials containing active hydrogen groups, suitable for powder spray applications.
2. Description of the Prior Art
Powder coatings are prepared from powdered resins applied by electrostatic or fluidized bed processes. In the former, the powdered resin is applied by electrostatic spray gun to a substrate of opposite charge which is then baked to fuse the powder particles into a smooth coating. In the latter process the preheated substrate is immersed in a fluidized mass of powdered resin so that the powder adheres to the substrate which is then baked to fuse the powder into a smooth coating. Powder coatings possess significant advantages over conventionally applied solvent finishes in economy, ease of application, single coat potential and absence of sagging. Moreover, air pollution is eliminated.
Thermoplastic and thermosetting resins may be applied by powder coating techniques. Thermosetting powders offer several advantages; namely, they require no primer, they yield tough flexible films of superior chemical and solvent resistance, they provide adequate protection to the substrate in thinner films, they accept higher pigmentation levels and they may be applied at lower curing temperatures.
Powdered epoxy resins have been successfully developed for powder coatings. Unfortunately, epoxy coatings weather poorly, rapidly losing gloss. Hence, for exterior uses, thermosetting acrylic or polyester powder coatings would be preferred. Attempts to produce such coatings have been based on systems which contain aminoplast crosslinking agents and acrylic or polyester resins with reactive hydrogen groups such as hydroxyl, carboxyl, or amide groups. To date, such systems containing conventional aminoplasts; for example, methylol and alkoxymethyl derivatives of ureas, melamines and benzoguanamines, have been unsatisfactory because of poor package stability. The poor package stability is associated with the plasticizing and cold flow effects of the conventional low molecular weight aminoplasts, causing clumping and sintering or fusing of the powdered resin. Clumping and sintering are undesirable since they impair fluidization of the powder during application to the substrate, they cause segregation of the powder into varying degrees of density, and they contribute to non-uniform coatings with decreased gloss. The cold flow may be avoided by cutting back on the quantity of aminoplast but with sacrifice in physical properties and solvent resistance of the cured film, or by the use of high molecular weight acrylic or polyester resins, an expedient which impairs fusibility of the powder and flow so that the final coating lacks gloss and coherence, or by the use of acrylic or polyester resins of high glass transition temperature so that the glass transition temperature of the powdered composition is increased but the cured film is embrittled and loses impact strength.
Thus, there is a need for powdered resin compositions comprising a blend of aminoplast crosslinking agents and acrylic or polyester materials containing active hydrogen groups, which have adequate package stability and flowability and which yield powder coatings which when cured have adequate physical properties and solvent resistance.